[deadbodyman]

Fast & firm is a good sealer for most seams but drip rails are just too much for it,if put on too thick it skins over and cracks or never dries...theres nothing wrong with your prep...You'll need something more rubbery (flexable).
I cant remember what Ive used in the past but someone can give you a brand name , if not your local paint supplier can help....

[Lizer]

I haven't used it on the drip rails yet, but the places where I noticed it was cracked was in the seam along the inner wheel house and trunk floor, and in the back along the rear trunk brace where it met the tail light panel. Keep in mind the car is just a rolling chassis and sits in the garage. However, it is perfectly intact in other places, like along the exterior seam of the cowl where it meets the firewall.

The 3M brushable seam sealer I use also is cracking in some places too, which doesn't inspire confidence in its long term efficacy...